Proximal tubules of the kidney are responsible for the reabsorption of both 90% of the filtered load of bicarbonate and 70% of the filtered fluid. Both of these processes are interrelated. Recent developmnet of methods for measuring bicarbonate concentrations in small volumes has allowed for accurate evaluation of bicarbonate transport in the setting of micropuncture and in vitro microperfusion. The latter method, because it allows for greater control of the acid-base status of the tubule environment and because it allows for examination of nephron segments not accessible to micropuncture, is well suited for detailed investigations of the regulatory influences of acid-base changes on proximal tubule transport. The studies proposed in this application have three major goals: First, to examine the mechanism by which renal proximal tubules reabsorb bicarbonate; second, to examine various regulatory influences on bicarbonate reabsorption; and third, to determine the relationships between bicarbonate reabsorption, net volume reabsorption, the reabsorption of sodium, chloride, phosphate, and various organic solutes. Proximal tubule segments from rabbits will be perfused in vitro and standard microperfusion methods will be supplemented by microelectrode measurements of intracellular voltage and pH. This combination of techniques will allow for analysis not only of transepithelial events, but also of transport events occurring across the luminal and peritubular cell membranes. Thus the effects of bicarbonate concentration, pH, pCO2, potassium, and peritubular oncotic pressure on proximal tubule bicarbonate and volume reabsorption can be examined in detail previously not possible. The results of these studies are expected to provide major new information on the mechanisms and regulation of proximal tubule function.